1. Field of the Invention
The present invention relates to a discharge lamp lighting control device for controlling the lighting of a discharge lamp that emits light through a discharge caused by a breakdown. More particularly, it relates to a discharge lamp lighting control device suitable for controlling the lighting of a discharge lamp intended for a headlight mounted in a vehicle such as an automobile, and an H-bridge circuit for use in such a discharge lamp lighting control device.
2. Description of the Prior Art
Conventionally, halogen lamps, in which a halogen gas is contained into an electrode tube, have become into widespread usage as automobile headlights. Halogen lamps provide an advantage that they can emit a higher light flux compared with filament bulbs. However, attempts have been made to make use of discharge lamps, such as high-voltage sodium lamps or metal halide lamps, which have been used as street lamps, as automobile headlights, instead of halogen lamps.
Referring next to FIG. 5, there is illustrated a block diagram showing a prior art discharge lamp lighting control device as disclosed in Japanese Patent Application Laying Open (KOKAI) No. 4-26002. In the figure, reference numeral 1 denotes a DC power supply, such as a battery, which can be mounted on an automobile, 2 denotes a DC/DC converter which converts the power supply voltage from the DC power supply 1 into a higher predetermined voltage, 3 denotes DC/AC inverter which converts the predetermined voltage from the DC/DC converter into an AC voltage, 4 denotes a discharge lamp, 5 denotes an igniter circuit connected between the DC/AC inverter 3 and the discharge lamp 4, for generating and applying an extra-high voltage to the discharge lamp when supplying the power to the discharge lamp by further raising the AC voltage from the DC/AC inverter 3, and 6 denotes a control circuit for controlling the DC/DC converter 2 so that the discharge lamp lighting control device supplies a constant power to the discharge lamp 4.
Furthermore, reference numeral 8 denotes a transformer having its primary coil electrically connected to the DC power supply 1 and its secondary coil electrically connected to the discharge lamp 4, for generating, at the secondary coil, a voltage which is higher than that applied to the primary coil, 9 denotes a switch connected, in series to the DC power supply 1, between the primary coil of the transformer 8 and the DC power supply 1, 10 denotes a resistor, which is disposed as a voltmeter, for measuring the DC voltage that appears at the secondary coil of the transformer 8 as the lamp voltage, 11 to 14 denote transistors connected between the transformer 8 and the igniter circuit 5, and 22 denotes a switching control circuit for performing an on/off control operation on the four transistors. The four transistors 11 to 14 make up an H-bridge circuit.
While the discharge lamp 4 does not light up, the two electrodes of the discharge lamp 4 are insulated from each other. When a lighting setup process of causing an arc discharge within the discharge lamp 4 is started in the insulated state, the control circuits 6 operates in such a manner that a high voltage of about 400 Volts appears at the secondary coil of the transformer 8 within the DC/DC converter 2. The switching control circuit 22 controls the H-bridge circuit so that the high voltage is supplied to the igniter circuit 5, just as it is. The igniter circuit 5 raises the high voltage up to a higher voltage of about 20 kV. When the high voltage of about 20 kV is applied to the discharge lamp 4, an arc discharge is caused to pass through the gas between the electrodes of the discharge lamp by a breakdown. The discharge lamp 4 then starts emitting light.
When such a breakdown is caused within the discharge lamp, the igniter circuit 5 stops the ignition operation and the DC/AC inverter 3 starts the switching operation. Accompanied by the breakdown, the voltage that appears at the secondary coil of the transformer 8 is reduced to a voltage dependent upon the rated voltage of the discharge lamp 4. The control circuit 6 then changes its control operation so that the discharge lamp lighting control device supplies a constant power to the discharge lamp 4. An AC voltage controlled to have a constant power is thus applied to the discharge lamp 4, and the discharge lamp 4 continues to emit light.
Controlling the lighting of the discharge lamp 4 with a constant power supply is intended for increasing the lifetime of the discharge lamp 4 from the viewpoint of the characteristics of the discharge lamp 4.
Conventionally, the four transistors 11 to 14 included in the H-bridge circuit are transistors with the high withstand-voltage characteristic of about 400 Volts. Accordingly, a problem with the prior art discharge lamp lighting control circuit is that the four transistors require a large space because heat radiation from the transistors must be considered and the large space occupied by the four transistors therefore prevents the downsizing of the H-bridge circuit and hence the device.
Especially, since the control circuit 6 measures the DC voltage that appears at the secondary coil of the transformer 8 by means of the resistor 10 and then controls the DC/DC converter 2 according to the measured DC voltage, the four transistors 11 to 14 that make up the H-bridge circuit should have identical characteristics. If they do not have identical characteristics, an AC voltage generated by the H-bridge might become unstable even though the DC voltage applied to the H-bridge circuit is controlled to remain constant. Therefore, four large-size transistors with the high withstand-voltage identical characteristic of about 400 Volts are employed as the transistors 11 to 14 in the prior art H-bridge circuit.
Another problem with the prior art discharge lamp lighting control device is that if the high voltage applied to the discharge lamp 4 becomes unstable, the discharge lamp 4 is brought into an unstable lighting state in which it cannot emit light having a constant luminous energy with stability.